This invention relates to a catalytic process for producing aqueous hydrogen peroxide safely from gaseous hydrogen and oxygen at a concentration above the lower flammability limit, and more particularly, it relates to such process which provides for the continuous production of an aqueous stream containing useable concentrations of hydrogen peroxide and an effluent gas stream containing residual hydrogen and oxygen at concentrations outside the flammable and explosive limits for hydrogen and oxygen mixtures.
The production of hydrogen peroxide by the direct reaction of hydrogen and oxygen in aqueous solution in the presence of a catalyst appears to have been first disclosed in 1914 in Hinkel et al, U.S. Pat. No. 1,108,752, and has been the subject of numerous patents to the present.
Over the years it has become well established that the reaction is best conducted at relative partial pressures of oxygen to hydrogen of at least 2/1, and that the selectivity of the reaction to produce hydrogen peroxide (rather than water) increases with increasing oxygen to hydrogen ratios. For example, U.S. Pat. No. 4,009,252 discloses reacting hydrogen with oxygen in an aqueous medium using selected concentrations a platinum-group catalyst and partial pressures of hydrogen and oxygen of at least 0.5 atmos. and 1.0 atmos., respectively, at oxygen/hydrogen ratios of 1.5 to 20/1, and preferably 2/1 to 10/1. U.S. Pat. No. 4,336,239, which relates to continuous a well as batchwise production of hydrogen peroxide from hydrogen and oxygen, discloses that it is preferred to carry out the process at oxygen/hydrogen ratios of above 5/1, with 12/1 and 15/1, as being generally more preferred ratios.
The reaction of hydrogen and oxygen to produce hydrogen peroxide is not without danger, however, since mixtures of hydrogen and oxygen are flammable, even explosive, at hydrogen concentrations above 5%, the lower flammability limit, which corresponds to oxygen/hydrogen mole ratios of 20/1 and below, and includes the generally preferred oxygen/hydrogen operating ratios. Accordingly, to minimize the risk of explosion or fire the art also teaches the use of a diluent gas, such as nitrogen helium, neon or argon, as disclosed in U.S. Pat. Nos. 4,009,252 and 4,661,337.
It has also been proposed to employ oxygen/hydrogen ratios that are well outside the flammable and explosive limits. U.S. Pat. No. 4,681,751, for example, discloses that it is preferred to employ oxygen/hydrogen ratios of 20/1 or higher to avoid the danger of explosion during continuous process runs. Similarly, U.S. Pat. No. 4,336,239, cited above, states the reaction can be carried out at oxygen/hydrogen ratios of 23/1-40/1 which are outside the limits of flammability and prevent an explosion hazard without requiring an inert diluent gas. Although providing a measure of safety, the use of inert gas diluents an high oxygen/hydrogen ratios are disadvantageous expedients. The result in unnecessarily high reaction pressures which necessitate the use of more costly high-pressure equipment. Further, oxygen/hydrogen mole ratios in excess of about 10/1 provide little or no improvement in the selectivity of the reaction to produce hydrogen peroxide.
Continuous operation of the direct combination process is highly desirable for the production of hydrogen peroxide. Continuous modes of operation described in the art, however, are not entirely satisfactory from a commercial standpoint. U.S. Pat. No. 4,279,883 describes a continuous process involving a stirred tank reaction system having a gaseous zone and a liquid zone, means for feeding reactant and diluent gases, means for feeding an aqueous liquid reaction medium containing dispersed metal catalyst, and means for the continuous removal of a spent gas composition and an aqueous liquid reaction product. Example 1 of the '883 patent shows the use of a gaseous feed mixture consisting of hydrogen, oxygen, and nitrogen at partial pressure of 5 atms., 49 atms., and 113 atms., respectively, corresponding to a nitrogen diluted oxygen/hydrogen ratio of 9.8/1.
The disclosed continuous process suffers in utilizing a relatively large ratio of nitrogen to the reactant gases which adds to the cost of the operation. It has also been found that in a stirred tank system, as above, dispersed metal catalyst tends to adhere to and creep up the walls of the reactor and the stirrer shaft into the gas zone where it can become exposed directly to the reactants in the gas phase. Should the catalyst dry out during the course of the reaction, there is the ever present danger that, in the absence of a diluent gas, the dry catalyst would catalyze of the hydrogen phase mixture. Thus, that prior art has employed diluent gas to guard against the possibility of fire or explosion during the exemplified continuous process.
The U.S. Pat. No. 4,336,239, cited above, discloses a continuous hydrogen peroxide process conducted in a tower reaction packed with catalyst and equipped with means for upward concurrent feed of hydrogen peroxide and reaction solvent. At the top of the reactor, there is a device for the removal of liquid samples, means for transferring the reactor effluent to a liquid gas separator, means for venting spent gaseous effluent and means for introducing a diluent stream of nitrogen. The patentees describe a series of runs in Examples 3, 4 and 5 utilizing hydrogen and oxygen over a wide range of oxygen/hydrogen ratios, i.e., from about 2.9/1 to about 30/1. Example 4 exemplifies oxygen/hydrogen ratios of 23/1 to 30/1, which are outside the limits of flammability or explosion.
It will be noted that the concentrations of hydrogen peroxide in the resulting liquid effluent are relatively low, ranging from 0.14 to 0.70 molar or about 0.5 to 2.4% by weight of the effluent. The composition of the spent gas vented from the gas-liquid separator is not described. The hydrogen peroxide production results, however, indicate that the residence time o hydrogen and oxygen in the tower reactor is insufficient to substantially lower their concentrations in the liquid reaction medium, so that where oxygen and hydrogen are fed to the reactor in ratios that are initially in the flammable-explosive range in those runs of Example 3 that do not employ a diluent as), these reactant components are still in the flammable-explosive range in the spent gas vented from the gas-liquid separator. This is further suggested by the presence of a diluent nitrogen feed means at the top of the lower reactor so that nitrogen can be fed to the gas-liquid separator as needed to maintain a "safe" gas composition in the gas zone of the separator and in the gas vented therefrom. This is still further suggested by (a) Example 3 run C which shows the use of oxygen-enriched air to provide a nonflammable hydrogen-containing feed composition, and (b) Example 4 which used oxygen/hydrogen feed ratios outside the limits of flammability or explosion.
The '239 process also suffers the disadvantage of involving liquid reaction media composed of or containing substantial proportions of organic solvents. These solvents not only add to the cost of the operation but result in hydrogen peroxide solutions having limited utility and marketability. Further, the organic components present the hazard of explosive peroxide buildup as disclosed in U.S. Pat. Nos. 4,009,252, 4,681,751, 4,772,458 and 4,389390. Although, as stated earlier, effluent gas containing unreacted hydrogen and oxygen can, if necessary, be rendered nonflammable and nonexplosive by use of diluent gas or excess oxygen, such uses only add both to the investment and operating cost of the process.
It is also known to conduct hydrogenation reactions, including for the production of hydrogen peroxide in the cyclic anthraquinone hydrogenation and oxidation process, in pipeline reactors, as disclosed in U.S. Pat. Nos. 3,423,176 and 4,428,923. The former reactor involves a plurality of upwardly directed elongated spaces (tubes) of smaller diameter alternate connected in series with a plurality of downwardly directed elongated spaces of larger diameter, the reactants being passed concurrently upwardly through the smaller diameter tubes and downwardly through the large diameter tubes. The above reaction system is not entirely satisfactory for the present purpose as it appears limited to relatively low flow velocities, i.e., up to 3 meters/second or about 9.9 feet/second.
More importantly, as pointed out in the '923 patent, phase separation of the gases/liquid reaction mixture can occur under certain conditions avoided in the hydrogen-oxygen reaction system, since oxygen would be present along with hydrogen in the separated gas phase, thus creating an explosion hazard within the reactor.
The '923 patent pipeline process is directed to the production of a hydrogenated anthraquinone intermediate to hydrogen peroxide. It utilizes a loop reactor made of tubes with the same nominal width, arranged vertically or horizontally and connected by curving tubes (elbows). The reactor length varies from 15 to 150 meters (about 50 to 500 feet), the inside diameter from about 350 to 700 mm (about 13.8 to 27.6 inches). Reaction flow velocities are greater than 3 and up to 10 meters/second (greater than about 9.8 and up to about 32.8 feet/second). preferably 4 to 7 meters/second (about 13 to 23 feet/sound). Such apparatus and process are designed, however, to eliminate pressure losses from the tube expansions and contractions in the '176 patent and to thereby increase reaction liquid flow velocity and consequently the production of the desired hydrogenated anthraquinone. Patentees provide no teaching relative to the direct combination of hydrogen and oxygen to form hydrogen peroxide or the problems of hydrogen-oxygen flammability associated with the direct combination process.